1. Field of the Invention
This invention relates to a control system for an internal combustion engine, which carries out auxiliary fuel supply for supplying fuel to a cylinder as auxiliary fuel over a predetermined time period within a time period from an expansion stroke through an exhaust stroke of the engine, so as to control a catalyst for reducing exhaust emissions to a predetermined state for causing the catalyst to exhibit its emission-reducing capability, and a control system for an internal combustion engine, which supplies fuel to a cylinder and performs EGR for causing part of combustion gases generated by combustion to be present in the cylinder, by an EGR device.
2. Description of the Related Art
Conventionally, a control system of the first-mentioned kind has been disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H11-107813. The internal combustion engine is of an in-cylinder injection type for directly injecting fuel into a cylinder, and a NOx catalyst is disposed in an exhaust pipe thereof, for reducing exhaust emissions. When the amount of a sulfur component adsorbed by the NOx catalyst is large, in order to cause the sulfur component to be released from the NOx catalyst to restore the emission-reducing capability thereof, the control system carries out not only normal fuel injection for the engine power output but also expansion stroke injection for injecting fuel during the expansion stroke. This causes unburned fuel to be contained in exhaust gases and be burned in the exhaust pipe, whereby the temperature of the NOx catalyst is raised to such a high temperature as enables the NOx catalyst to release the sulfur component adsorbed thereby. Further, the total sum of the amount of fuel injected for the engine power output and the amount of fuel injected during the expansion stroke is calculated such that exhaust gases are made rich, and the ratio between the two amounts is determined according to temperature required by the NOx catalyst and the engine speed. The control system causes the sulfur component to be released from the NOx catalyst by the above two control operations for adding unburned fuel and making exhaust gases rich, to thereby prevent the NOx catalyst from being degraded in the emission-reducing capability thereof.
In the conventional control system, however, as described above, the ratio between the amount of fuel injected for the engine power output and the amount of fuel injected during the expansion stroke is determined according to the temperature required by the NOx catalyst and the engine speed, during execution of expansion stroke injection, and therefore the amount of fuel injected for the engine power output sometimes becomes too small or too large with respect to the intake air amount at the time. In such a case, there is a fear that fuel economy is degraded during execution of expansion stroke injection, and the engine power output cannot be obtained, as desired, resulting in degraded drivability.
Further, when the conventional control system is combined with an EGR device for recirculating part of exhaust gases into the intake pipe of the engine, and expansion stroke injection and the exhaust gas-recirculating operation of the EGR device are performed concurrently, there occur the following inconveniences: In this case, part of unburned fuel caused to be contained in exhaust gases by expansion stroke injection flows into the cylinder after being recirculated into the intake pipe by the recirculating operation of the EGR device. This causes a substantial increase in the amount of fuel burned in the cylinder. The conventional control system only determines the total sum of the amount of fuel injected for the engine power output and the amount of fuel injected during the expansion stroke such that exhaust gases are made rich, so that if the amount of fuel burned in the cylinder substantially increases, the output torque of the engine can be fluctuated, resulting in degraded drivability.